COT (constant on-time) control method is widely used in power supply area due to its simple structure and excellent transient response. Nevertheless, for switching converters with COT control, there may be a sub-harmonic oscillation at the output voltage because of the insufficient ESR (equivalent series resistance) of the output capacitor.
To avoid this sub-harmonic oscillation, a compensation signal in phase with the inductor current is often employed. FIG. 1 illustrates a schematic of a prior COT switching converter 100, wherein a ramp compensation generator consisting of a resistor Rramp and a capacitor Cramp is coupled to the inductor L in parallel. The compensation signal RAMP generated by the ramp compensation generator is added to a feedback signal FB to avoid this sub-harmonic oscillation.
However, the compensation signal may affect the average value of the feedback signal FB and results in poor load and line regulation. Also, the resistor Rramp coupled between the switching node SW and the feedback signal FB affects the load transient quite much. When the switching converter works in CCM mode, the ringing and noise from the switching node SW may be conducted to feedback signal FB through the ramp compensation generator when the switches M1 and M2 are turned OFF, and may cause the double pulse problem. In addition, the parameters of the ramp compensation generator shown in FIG. 1 are difficult to determine under the different input voltages, output voltages and switching frequencies.